1. Field of Invention
The present invention relates to a chromating method of zinc electroplated steel sheet, zinc-nickel electroplated steel sheet, and Galvannealed steel sheet (these steel sheets are hereinafter referred to as the zinc-based plate steel sheets). Particularly, the chromate film formed by the method of the present invention exhibits, even after drying at low temperature and/or for a short time, improved corrosion-resistance, alkali-resistance, welding-resistance and coating-performance (i.e., adhesion and corrosion-resistance of painted film).
2. Description of Related Arts
Heretofore, the chromating liquid has been an aqueous solution containing chromic acid and bichromate, and enabling the formation of chromate film which is difficult-to-dissolve when treated by acid or alkali.
Various methods for forming the chromate film have been proposed. The previously proposed methods as well as their merits and drawbacks are hereinafter described.
Japanese Unexamined Patent Publication No. 50-158535 relates to a method for forming a difficult-to-dissolve chromate film and discloses the chromating liquid, which contains chromic acid anhydride, phosphoric acid and water-soluble or water-dispersed polymer. Seventy percent more of the hexavalent chromium ions contained in the chromating liquid is reduced by a reducing agent, such as ethylene glycol or the like. The polymer contained in the chromate film improves the dissolving resistance, corrosion-resistance and coating-performance, but disadvantageously impairs the weldability.
Japanese Patent Publication No. 61-58552 discloses the chromating liquid, which contains chromic acid, chromate-reducing product and silica sol.
When the chromate film is formed on a zinc-based plated steel sheet by the above-mentioned chromating liquid and is then worked and painted, specially hexavalent chromium ions are liable to dissolve away from the chromate film because of alkali washing before the painting.
As the result, the corrosion-resistance of the chromate film is deteriorated.
Japanese Unexamined Patent Publication Nos. 58-22383 and 62-83478 disclose a method to reduce the hexavalent chromium ions contained in the chromating liquid by a silane coupling agent. The chromate film produced by the methods disclosed in said publications exhibits excellent adhesion of the paint coating. However, the chromate film produced by Japanese Unexamined Patent Publication No. 58-22383 is free of phosphoric acid and hence exhibits poor alkali-resistance. The chromate film produced by Japanese Unexamined Patent Publication No. 62-83478 exhibits also poor alkali-resistance. In the conventional chromating method described above, the ingredients are considered to have the following tendencies. Polymer and silica enhance corrosion-resistance but impair alkali-resistance. There is a tendency for the hexavalent chromium ions in the chromating liquid to be reduced by the silane coupling agent, with the result that the concentration of the hexavalent chromium ions gradually decreases during the film formation, thereby destabilizing the corrosion-resistance of the chromate film.
Since the performance of the chromate film is strongly dependent on the drying condition, the chromate film which is formed by any one of the various methods proposed as above exhibits poor performance when dried at a low temperature and/or for a short period of time.